After exiting blender, texture I was painting shows up black
In Blender, when you texture paint in Blender, it doesn't save the images when you save your file or close Blender (unless you have Blender pack, or save, all images into the .blend file).

It keeps the slot (that's what you observed) because the texture is being pointed to by your materials, but if you don't save it externally, you've lost your textures.